1. Field of the Invention
The present invention relates to a method for fabricating cylindrical and prismatic rechargeable metal-air batteries and their structure, which depends on micro pumps to create air draft passing the gaps between packs of electrodes and separators, and the micro pumps are turned on and off by an in-cell actuating system.
2. Related Art
The flourishing telecommunications have wired people around the world. In the near future, cellular phones will become daily tools rather than luxuries. As internet proliferates and prevails at every level, more and more people will connect the web via portable electronic devices such as cellular, notebook and PDA (personal digital assistant). All the activities in conjunction with increasing power need of the devices, more than ever, depend heavily on rechargeable batteries.
The fashion-trend of the portable electronic devices is an incessant reduction in size and weight. It imposes great challenges on the battery manufactures. They have to produce smaller and thinner batteries, and yet the capacity and other performances of the batteries are not to be compromised. In designing a battery, developers must consider its energy density (Wh/l) and specific energy density (Wh/Kg). The former has more impact than the latter, as the reduction in size is easier to observe than that in weight. Currently, rechargeable lithium ion (Li+) and nickel-metal hydride (Ni-MH) batteries are the two principal energy sources for many portable electronic devices. However, their dominance might be replaced by lithium polymer batteries someday. The new batteries hold some advantages including size and weight.
ZAB (Zn-air battery) should be another promising candidate for the rechargeable battery of next generation. Firstly, ZAB is very economical as Zn is an essential, cheap metal and the cathode of ZAB reacts on oxygen that is abundant and free. Secondly, ZAB is a safe and environmentally friendly battery. Thirdly, comparing with Li+ and Ni-MH batteries, ZAB is superior in energy densities: Z-A (200 Wh/Kg), Li+ (115 Wh/Kg) and Ni-MH (70 Wh/Kg); Z-A (276 Wh/l), Li+ (270 Wh/l) and Ni-MH (240 Wh/l). These numbers certify ZAB as a legitimate contender as the portable energy source.
Nevertheless, a rechargeable ZAB acceptable to the portable electronic industry is overdue. There are many technical hurdles need to be crossed before a viable secondary ZAB for 3C (communication, computer and consumer) applications is realized. Most of the technical difficulties are studied and addressed with feasible solutions, except one major problem is unsolved. The problem is the form factor that prevents ZAB from being made in cylindrical and prismatic forms as the commonly seen Li+ and Ni-MH batteries. The form factor is related to the air supply to the cathodes of ZABs when they are in cylindrical or prismatic form.
U.S. Pat. No. 5,569,551 discloses an electrically rechargeable ZAB using a Zn anode sandwiched between two air cathodes. While U.S. Pat. No. 5,554,452 reveals another electrically rechargeable ZAB module consisted of six-pack of batteries. Both works arranged the cathodes adjacent to air inlets. Because of the orientation, only two pairs of electrodes are allowed in one battery. In order to enhance the energy density as well as the power density, the resulted batteries are often bulky and heavy. U.S. Pat. No. 4,885,217 taught the use of a polymer web or net for constructing air cathodes. The net is sealed within the cathode to form an air pocket. Nevertheless, the spacers are rigid and the design can not generate cylindrical batteries.
The management of air of ZAB for a portable electronic device can be seen in U.S. Pat. No. 5,888,664. The air pathway is installed outside the batteries, and the module is bulky. The present invention devises an in-cell air management. When ZAB is configured in cylindrical or prismatic form, the air conduit can be constructed inside the battery similar to the water channel in U.S. Pat. No. 2,988,587. ""587 taught a water-activated magnesium reserve battery. The water channel for providing water to the cathodes is created by means of glass beads embedded on the cathodes. The present invention utilizes spacer sheets containing protrusions on forming air pathway during the fabrication of batteries. Micro fans are implemented in the middle of both end caps of batteries to create air draft in the pathway. The in-cell air management can effectively direct air to all layers of cathode.
The present invention provides an in-cell air management for metal-air batteries using the combination of:
1) spacer sheets with protrusions to form air pathway for the cathodes; and
2) micro pumps (fans) to create air draft in the pathway.
With the in-cell air management, metal-air batteries can be fabricated in cylindrical and prismatic forms. The gap-forming spacer sheets can be prepared via screen printing method or injection molding method of protrusion pattern on porous and thin substrates such as polypropylene, polyamide, polyethylene oxide, polyethylene terephthalate, polyacrylamide and polyurethane. As known to those skilled in the art, the protrusion materials may include epoxy, acetal, acrylic and urethane. If screen printing method is used, the protrusions are printed on the substrates and are subsequently cured by heat, radiation or moisture. While injection molding method can integrate the protrusions and spacer sheets in one step.
The spacer sheet is concentrically winded with other sheets of cell component such as electrodes and separator sheets to form the cylindrical batteries. The spacer sheet is placed with the protrusions (dots) against the cathode sheet. In making prismatic ZAB, the spacer sheet and cell-components sheets of the desired dimension are stacked in sequence with the protrusions against the cathode sheets. Because of the presence of protrusions, air pathway is formed by each layer of the cathode sheet.
Henceforth, it is thus an object of the present invention to provide air pathway for the cathodes of metal-air battery in close environments. The air pathway allows more pairs of electrodes per cell. The utilization of battery space is greatly enhanced. The energy density, power density, and working voltage of the resulted batteries are consequently improved.
It is another object of the present invention to impose micro pumps at both ends of metal-air batteries for creating air draft inside the batteries. Air is used by the cathodes of metal-air battery to generate electricity during discharge. The air pathway and micro pumps form the in-cell air management.
It is a further object of this invention to use the in-cell air management to fabricate metal-air battery in cylindrical and prismatic forms. The aforementioned shapes are the two most efficient configurations of batteries for portable electronic devices. With the in-cell air management, the form factor limitation on metal-air battery is lifted.
It is another further object of this invention to use the in-cell air management to control airflow in the batteries. As the air supply to the cathode is ceased, the chemical reactions of metal-air battery will stop. In theory, the battery can be preserved indefinitely. In other words, the in-cell air management is used to prolong the shelf life of metal-air battery.
It is a yet further object of this invention to devise an actuating system for the actuation of micro pumps. The actuating system includes a second cathode, which does not react on air, and an energy storage device such as supercapacitor. When a load is in request, metal anode works with the second cathode to provide nominal current to the supercapacitor. The latter amplifies the energy output to power the micro pumps to turn on metal-air battery. The in-cell actuating system is devised to impart metal-air battery a self-sustained breathing battery.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.